The style of bicycle commonly known as a mountain bicycle is designed and intended for use off-road on trails and rough ground. Since this type of bicycle is routinely ridden vigorously over rough terrain, rocks and other obstacles, the frame structures and related components are subject to higher loadings and greater physical abuse than their road-going counterparts. In order to endure this abuse, all of the structures and components of the bicycle must be designed and manufactured to be very sturdy. The front fork of the mountain bicycle, in particular, is subjected to high loadings as the front wheel impacts against obstacles or lands after jumps. The structural integrity of the front fork is critical in that a failure of this component almost ensures a serious fall for the rider.
In light of these considerations, conventional mountain bicycle forks are typically built of metal with thicker sections than comparable road-going bicycle forks and, as a result of this, they are substantially heavier. A light weight bicycle is desirable because any excess weight requires additional energy to accelerate or climb hills and it slows the response of the bicycle to rider control inputs. Thus, in the case of conventional mountain bike forks, the additional weight of structure for necessary strength is detrimental to the overall performance of the bicycle.
Conventional mountain bicycle forks typically have an inverted Y-shape configuration with the depending legs (known as fork blades) extending from the attachments at either side of the front wheel axle up to a crown structure which forms the connections between the fork blades and the steerer tube. The steerer tube then extends upward through the head tube of the bicycle frame where it pivots in the head set bearings. Handle bars are then connected to the steerer tube through a stem assembly. This configuration takes the loads imposed through the front wheel up through the fork blades and concentrates them in the steerer tube and in the joint where it intersects the crown. The front wheel and fork blades are thus cantilevered off of the steerer tube by a single crown connecting structure.
To sustain the bending stresses imposed by this cantilevered load, the steerer tube must be extremely strong. Conventional head set bearings constrain the diameter of the steerer tube to 1 inch or 1.25 inches. As a result of this, the conventional steel steerer tube must be thick walled and consequently heavy. Examination of the weights of the individual components which make up the fork assembly shows that the steerer tube is the heaviest single component. Substitution of less dense materials, such as aluminum, titanium or fibrous plastic composites, for the steel of the steerer tube does little to decrease the weight because the lower density materials also have lower moduli of elasticity. Thus, for the same outside diameter they must be thicker walled to have the same stiffness. This negates the advantage of their lower density.
Another important design consideration in making the steerer tube is the economy of having it interface with conventional handle bar stems which require a standard internal tube diameter. Thus, this consideration may prohibit the use of steerer tubes with a thicker wall, at least at its interface with the handle bar stem, in a front fork designed for industry-wide use.
The single crown connection of conventional mountain bicycle forks is also a critical link between the front wheel and the bicycle frame because it must take the relatively high bending loads from the blades and transfer them to the steerer tube through relatively small cross-sectional areas. These high loads are further concentrated at the relatively small areas of the joints between the different fork components and such concentrations result in very high stress loadings across these joints. Stress concentrations at such interface areas may be further aggravated because of differences in materials, such as where a part made of another metal meets the steel of a steerer tube or a welding or braising flux is provided between two parts of different alloy compositions. Welding or braising also adds undesirable weight to the fork. Different metals also have different thermal expansion coefficients which may further aggravate stress concentrations.
Attempts were made in the early development of bicycles to reduce the stresses in the steerer tube. One such attempt is disclosed in U.S. Pat. No. 593,814 granted to Louis De Rango on Nov. 16, 1987. Mr. De Rango proposed an H-shaped structure which extended the fork blades up past the lower crown to about the race of the top head set bearings, where the blades formed an attachment at the top of the bicycle head tube. This configuration significantly reduced the cantilevered loading condition as compared to the steerer tube connections of conventional Y-shaped forks. In De Rango, some of the bending stresses are carried in the upper portion of the fork blades and as a result, the bending stresses in the steerer tube are reduced significantly. The steerer tube can then have a much thinner wall and thus be a lighter structure. In fact, De Rango eliminated the steerer tube entirely in favor of small opposing stubs for carrying the head set bearings. Motorcycles commonly use a configuration much like the De Rango design except that a one piece steerer tube is employed instead of small opposing stubs. These configurations provide strength and stiffness far greater than that if the fork tubes are terminated at the lower crown, such as on conventional bicycles.
Such H-shaped structures of the prior art, with or without a full steerer tube, involved some of the same disadvantages as the inverted Y-shaped forks. For example, excessive concentrations of stress may still result in such prior art H-shaped structures because they still involve the use of crown members of small cross-section for connecting the blades at the top and bottom of the steerer tube or equivalent steerer structure. Furthermore, the H-shaped forks still required assembling numerous individual parts, often of different materials, and where these parts were welded or braised together, significant stress concentrations still occurred, sometimes leading to structural failure. The welding and braising also results in undesirable increases in weight. In some instances, the cross bar of the "H" may be clamped, instead of welded, to an intermediate portion of the extended blades. In these instances, the relatively small area of the blades engaged by the clamping device may result in undesirable high levels of stress concentration. Such clamping devices also may slip and cause misalignment of the blades under the high bending loads and different force vectors to which a front fork is subjected as the bicycle is ridden over rough terrain. Furthermore, such clamping devices often involved more weight than a welded or braised connection.